The goal of this project is to purify and characterize the transport system(s) which is (are) responsible for the secretion of organic anions, typified by PAH, into the renal proximal tubule of the rabbit. A new preparation for basal lateral membranes which makes use of a zonal rotor is described. This procedure allows for a purification of basal-lateral membranes of nine- to thirteenfold, compared to a homogenate of renal cortex. Probenecid-inhibitable transport has been documented in these basal-lateral membranes. Brush-border membranes, prepared by divalent cation precipitation, seem to have at least a probenecid-inhibitable binding site for PAH. Detailed kinetic studies are being performed by a method that measures accurately initial rates. The physical-chemical requirements for transport and binding are being determined. Specific ligands which could substitute for PAH in an affinity column are being sought.